1. Technical Field
The present invention generally relates to etching techniques and, more particularly, to a method for improving the etching resistance of photoresist masks.
2. Discussion
Photoresist masks are used during etching processes to enable the formation of small features such as via holes in the underlying material. As microelectronic devices have increased in complexity, feature sizes have become smaller and smaller. Using present techniques, high aspect ratio features are relatively difficult to etch using plasma dry etching and/or wet chemical etching processes.
With respect to plasma dry etching, the photoresist erodes so that mask features, such as via hole openings, increase in size. Additionally, line widths shrink in size. During wet chemical etching, the photoresist undercuts, causing distorted circuit features. In high density circuitry (e.g., VLSI or ULSI) these changes in feature size make it more difficult to predict actual circuit performance with over-etched transmission line widths.
An additional problem with densely packed circuit layouts is that overetching of closely spaced via hole openings causes the via hole openings to overlap with each other. This results in circuit reliability problems. Such circuit reliability problems include changes in inductance due to intersecting via holes.
In an effort to overcome the foregoing problems, techniques such as hard baking photo resists in a convection oven or on a hot plate and deep ultra violet curing have been employed. Unfortunately, these techniques have not provided sufficient deterrents to the above problems. For example, conventional hard baking and deep ultraviolet hardening methods simply cross-link the photoresist polymer which does not make the photoresist highly resistant to subsequent harsh plasma etch environments. The deep ultraviolet method also requires special equipment which adds cost to processing. The ultraviolet method may also present ultraviolet radiation hazard to the process work environment.
In view of the foregoing, a new technique for improving etch processing is needed.
A plasma photoresist hardening technique is provided to improve the etch resistance of a photoresist mask. The technique involves the formation of a thin passivation layer on the photoresist mask which substantially slows down the etching rate of the photoresist material. Advantageously, this technique allows preservation of critical dimension features such as via hole openings and transmission lines. In one embodiment of the present invention, the technique hardens the surface of the photoresist film by both chemically and physically bonding halogenated hydrocarbons with cross-linked photoresist polymer. This results in a passivation layer which is highly resistant to harsh plasma etch environments.